I have the damnest time doing this - either use too much force, and score the whole way through the fat, or, with less pressure, I barely even make a dent in the fat. I used to think it was my knives - but the one I used today easily sliced through a tomato, so I think it should be sharp enough. I've also tried a serrated knife, but that's not worked any better.
One thought that I've not tried is to freeze them a bit, so that the fat firms up - presumably that would make it easier? These breasts were at room temperature, as I had just finished cutting up the duck.
Any and all tips welcome!